The fresh picked or harvested quality attributes of harvested fresh fruits, vegetables and horticultural products such as cut flowers must be maintained as much as possible for as long as possible to ensure consumer acceptability. Quality deterioration of harvested fresh fruits, vegetables and horticultural produce is caused by plant tissue enzyme reactions including respiration, ripening and senescence, through microbial growth and through water loss from the plant tissue. Methods of inhibiting the deteriorative enzyme reactions, and the growth of yeasts, molds and bacteria include the reduction of the produce temperature to between 1.degree. and 12.degree. C., and the creation of a low O.sub.2 /high CO.sub.2 modified atmosphere (MA) around the produce. Water in fruits and vegetables can be lost readily under low relative humidity conditions with the consequential detrimental result of skin wrinkling, wilting and reduction in crispness. The rate of water loss from fresh produce can be restricted by storing the produce in closed package systems consisting of walls with low moisture permeability.
Modified atmosphere packaging (MAP) of fruits, vegetables and horticultural products is a process involving:
(1) Performing required pre-packaging conditions and treatment of the produce; PA1 (2) Packing the produce in a gas-permeable package system; PA1 (3) Introducing a gas comprising a predetermined ratio of CO.sub.2 and O.sub.2 into the headspace of the package system to create a modified atmosphere, or retaining existing air in the headspace of the package system; and PA1 (4) Closing and sealing the modified atmosphere package (MAP) system. PA1 (1) Maintain definitive beneficial equilibrium levels of CO.sub.2 and O.sub.2 in the headspace within the package; PA1 (2) Obviate gas pressure build-up within the package system; PA1 (3) Minimize moisture loss from produce held in the package system; PA1 (4) Prevent produce crushing and bruising; and PA1 (5) Maintain structural strength of the walls of the package system by inhibiting water migration from the interior of the package into the walls of the package system.
During storage of the MAP system, the fruits, vegetables and horticultural products convert O.sub.2 from the headspace to CO.sub.2 through the natural respiration process of the produce with the result that the O.sub.2 content in the headspace decreases while the CO.sub.2 content increases. An effective MAP package system for fresh produce regulates the influx of O.sub.2 relative to the efflux of CO.sub.2 from the package headspace to achieve and maintain a suitable modified atmosphere equilibrium in the headspace around the stored produce. This establishes an optimum environment for retention of the quality attributes of the fresh produce and reduction of detrimental microbial growth in the produce.
While properly controlled low O.sub.2 levels and elevated CO.sub.2 levels in the headspace around a fresh fruit, vegetable or horticultural commodity reduce the respiration and ripening rates of the fresh produce, and the growth of spoilage organisms (spoilogens), unsuitable modified atmospheres enveloping the produce in a package system can induce physiological damage to the fresh produce, prevent wound healing, enhance senescence and cause off-flavour formation of the produce. Oxygen levels of about 1% can suppress the development of spoilogens. Carbon dioxide levels of 5% or more can suppress the development of spoilogens. O.sub.2 levels lower than 1% bring about anaerobic respiration and off-flavour development, whereas CO.sub.2 levels of about 10% or higher inhibit spoilogen growth but, on the downside, may cause tissue damage to CO.sub.2 -sensitive commodities.
Package systems for MAP must be carefully designed and constructed from specific packaging materials to meet the following requirements:
Corrugated paperboard boxes and cartons are used commercially for the storage and transport of fresh fruits, vegetables and horticultural commodities. Advantages of corrugated paperboard boxes and cartons are relatively low cost per unit volume, low thermal energy wall conductivity, impact absorbing ability to prevent bruising of the packaged commodities and ease of disposal of the used package at the receiving end. However, conventional corrugated paperboard has a very high gas and moisture permeability and as such is unsuitable for modified atmosphere packaging of fresh fruits, vegetables and horticultural commodities.
Since gas and moisture permeabilities of package components of MAP systems are critical parameters, conventional corrugated paperboard has been modified to include gas and moisture controlling polymer films. Plastic polymeric films have been developed so that a specific gas permeability requirement can be met with a single plastic film or a multifilm combination, with or without vent pinholes.
In 1960, Eaves (J. Hort. Sci. 37:110, 1960) reported the use of gas-permeable, flexible polymeric barrier film as a package system for extending the life of fresh commodities. Tomkins (J. Appl. Bacteriol. 25:290, 1962) used polymeric film-covered trays to determine their effectiveness in establishing equilibrium MA around apples. Prior art on the use of bags made from polymeric gas permeable films such as polyethylene and polyvinylchloride for prolonging of shelf-life of stored fruits and vegetables, is exemplified by U.S. Pat. No. 3,450,542, Badran, U.S. Pat. No. 3,450,544, Badran et al., and U.S. Pat. No. 3,798,333, Cummin. A more complex package system has been described by Rumberger in U.S. Pat. No. 3,630,759. There, an inner plastic pouch containing the produce is enveloped by an outer pouch containing an atmosphere of less than 15% O.sub.2. Both pouches are to be constructed from gas-permeable films.
U.S. Pat. No. 5,575,418, granted Nov. 19, 1996, Wu et al., discloses an invention relating to novel package systems for refrigerated modified atmosphere packaging of fresh fruit, vegetables and cut flowers. More particularly, the invention relates to the design, construction, closure, sealing and use of gas-permeable corrugated paperboard package systems for prolonging the storage life of fresh fruits, vegetables and cut flowers under modified atmosphere in the headspaces of the closed package system. The patent discloses a corrugated gas permeable paperboard comprising: (a) a first layer of Kraft paper; (b) a layer of polymer having a gas permeability which permits gas to be transmitted through the polymeric film at prescribed levels; (c) a second layer of Kraft paper, said first and second layers of Kraft paper sandwiching the polymer between them; (d) a corrugated fluting; and (e) a third layer of Kraft paper affixed to the corrugated fluting.
It has been noted from many field trials that distribution chains currently used for fruit and vegetable produce and horticultural products such as cut flowers do not necessarily provide adequate temperature control to ensure optimum produce and flower shelf life. Pallets and/or boxes stored at points along the distribution chain can be subject to unacceptable temperature rise causing the contents to increase their respiration rate, thereby leading to a shortening of the life of the fresh produce and flowers, both in terms of microbiological activity and sensory quality. Furthermore, when the MA package is subjected to a rise in temperature, the gas permeability rate of the polymer lining in the package will increase and thus allow higher levels of oxygen into the box. This promotes rapid decay of the fresh produce or cut flowers. There is therefore a strong need in the fresh produce or flower packaging and distribution industry for a MAP that will not only protect the fresh produce by controlling gas transmission rate into and out of the package but also provide temperature abuse resistance. Cut flowers generally have low respiration rates so the natural build up of respiration heat inside the MA box is minimal. The box is more likely to pick up heat from the surrounding conditions. The need to provide thermal protection is particularly important where fresh vegetable and fruit commodities are air freighted since the aircraft and airport apron handling operations are rarely temperature controlled. In certain parts of the world, for instance, it is common for MAP loaded pallets to sit in tropical conditions for hours waiting for the aircraft to be loaded.